This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS.
ABSTRACT. This study was unde rta ken to determine the sources of debris and methods of tra nsportation and d eposition in a nd on a small cirque glacier. Data were collected on the amount of debris, s tone roundness, the presence o f striations and polish, and till fa bri c. Li ch enometry gave r elative ages of the tills, and suggests that the Gannett Pea k till is of at least three ages and probably overlies Audubon till.Debris originating from subglacia l e rosio n ca n be diffe re ntia ted from that from rockfall or avalanches on stone roundn ess, polish and stria tions. A m ax imum of 70 % of the present glacial load derives from subglacial erosion, as compa r ed to 88% during the Gannett Peak stade. Rockfall rates are 35-50 m 3 /yea r at present and were 290-485 m 3 /year during the Gannett Peak sta d e. D ata on present-day processes and on the volume and age of Gannett P eak mora ines ca n b e u sed to make compa risons on present a nd past ra tes of d enudation . The d enudation r ate in the cirque at present is 95-165 mm / l 000 year ; in the past it was 4920-8 160 mml 1 000 year. The d e nuda tion rate and the glacial effects on d e bris are comparable to rates from other gl acia l areas and effec ts on d ebris carried by vall ey glaciers and continental ice sheets. REsuME . Origine, transport et depot de blocs su r le Arapaho Glacier, Front Range, Colorado , U.S.A. Cette etude a ete entreprise pour connaitre les origines et les modes de transport et de depot d e blocs rocheux sur un petit glacier de cirque e t a l'interieur du glacier. Les echa ntillons o nt ete ramasses p a rmi les blocs, pierres polies, avec stries et poli gl aciaire, en provenan ce d ' une struc ture morainique. L 'etud e d es lychens donne l' age relatif des moraines e t suggere que la mora ine de Gannett P eak est du plus p e tit d es 3 ages glacia ires, et probablement recouvre la moraine Audubon.Des blocs prove na nt d'erosion sous gl aciaire peuvent e tre diffe renties de ceux provenantde chutes de roch ers ou d'avala nches p a r la form e arrondie, le poli et les stries. Un maximum d e 70 % du materiel glacia ire actuel provient d 'erosion sous-glaciaire alors que pend a nt la periode Gannett P eak la proportion es t de 88% .Le taux de chute d e rochers es t de 35 a 50 m 3 par a n a l'he ure actuelle, et il etait d e 290 a 485 m 3 par an p e ndant la period e Gannett Pea k. L es procedes rel atifs aux informat ions actuell es, volume et a ge d es moraines de la p e riode Gannett Pea k, p euvent etre utilisees pour fa ire des compara isons entre les taux d 'erosion du prese nt e t du passe. L e taux d'erosion dans le c irque glaciaire es t a l'heure actuell e de 95 a 165 mm par 1000 ans. D ans le passe, il e tait de 4920 a 8 160 mm par 1000 ans. Le taux d 'erosion et les
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